<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>border_radius</title>
    <style>
        div{
            width: 200px;
            height: 200px;
            border: 1px solid red;
        }
        #one{
            border-radius: 100px;
        }
        #two{
            width: 100px;
            height: 30px;
            border: 1px solid red;
            border-radius: 15px;
        }
        #three{
            width: 30px;
            height: 100px;
            border: 1px solid red;
            border-radius: 15px;
        }
        #four{
            background-color: red;
            border-radius: 50% 50% 50% 0;
        }
    </style>
</head>

<body>
    <div id="one"></div>
    <br>
    <div id="two"></div>
    <br>
    <div id="three"></div>
    <br>
    <div id="four"></div>
</body>
</html>